Disc drives read and write information along concentric tracks formed on surfaces of one or more discs. The discs are typically mounted for rotation on the hub of a spindle motor. Axially aligned tracks on the disc surfaces are collectively referred to as a cylinder. Each of the tracks is divided into a plurality of sectors. Each of these sectors usually includes a servo field and a data field that are contiguous. The embedded servo fields are utilized by a servo sub-system to position an array of vertically aligned heads over a particular cylinder. The servo field of each sector includes a sector timing mark to verify receipt of the sector and to establish the timing between sequential sectors of a track.
In current disc drives, the servo fields are written onto the discs in-situ (that is, after the disc is mounted on the spindle motor of a disc drive) when the disc drive is manufactured and thereafter simply read by the disc drive to determine position during operation. Servo fields are written using constant frequency signals across the tracks. Thus, radially aligned servo fields occupy different linear distances in their respective tracks, but radially aligned servo fields occupy an identical timing window in each track (the angular velocity of each track is identical). Typically, in disc drives with in-situ written discs (in-situ written drives) there is a degree of misalignment between corresponding sectors (sectors that substantially overlap in a vertical direction) on different disc surfaces. This misalignment between corresponding sectors on different disc surfaces results in sector timing marks of corresponding sectors being detected by their respective heads at different time instants. For example, if the timing mark of a first sector of a first track on a first disc surface is detected at a first time instant during a particular disc revolution, the timing mark of a corresponding first sector of a first track on a second disc surface would be read at a second time instant during the same disc revolution due to the misalignment between the corresponding sectors. The difference in time between the first time instant and the second time instant is referred to as timing skew. The timing skew is constant between all corresponding sectors of any two different disc surfaces in in-situ written drives. Consequently, in in-situ written drives, the nominal timing skew value is stored in memory and utilized during head switch operations to open a timing mark search window near a timing mark that a destination head needs to detect. Thus, in in-situ written discs, the timing mark search window can be kept relatively narrow during a head switch operation.
To meet the demand for greater recording density in disc drives track writing is undergoing a fundamental change. In the near future, manufactured disc drives will include discs with tracks that are pre-written onto the disc before the discs are mounted on the spindle motor of the disc drive. When such discs with pre-written tracks (pre-written discs) are mounted on a spindle motor of a disc drive, there is a certain amount of misalignment of the track centers of the disc and the spin axis of the spindle motor. Because of such disc mounting tolerances, the tracks will be eccentric to the spindle axis of the spindle motor. One result of this servo track eccentricity is that the timing relation between heads (timing skew) is not constant as in the case of in-situ written discs but varies in a sinusoidal fashion. The magnitude of the sinusoidal timing skew has been found to be radially-dependent, that is, it varies coherently across the surfaces of the disc from the outer diameter (OD) to the inner diameter (ID). If the nominal timing value for the timing skew used in in-situ written discs is also used in the same manner for disc drives having pre-written discs, the timing-mark search window would have to be widened to account for the variation in the timing skew. Widening of the timing mark search window is undesirable since it increases the opportunity for false reads of the timing marks.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.